Apple informed dealers this week that some third-party memory expansion cards designed for the PowerBook 140 and 170 models will not fit properly in the just-introduced PowerBook 160 and 180 computers. Although the 160 and 180 models have the same specifications for their memory expansion slots, some third-party memory manufacturers have reportedly offered cards that don't quite meet the specs.

The notice said that the problem occurs because of sponge heat sinks on certain chips in the 160 and 180, limiting the empty space near the memory expansion connector. The sponge heat sinks are in one of several "keep out" areas published by Apple in its specs so that third-party developers will know where their components can "live" and where they must not. Because of the apparent difficulty in designing a card that includes more than 4 MB of RAM and still fits inside the allocated space, some manufacturers have sold 6 MB upgrade cards that, although they fit inside the 140, 145, and 170 without difficulty, take up more than their specified amount of space.

Vendors whose memory products don't fit will need to take a closer look at the PowerBook developer specs (an updated copy of which has been sent to hardware developers) and reconfigure their cards to fit inside the newer machines. In the meantime, early purchasers of PowerBook 160 and 180 computers will need to be careful that any memory upgrades they buy are specifically guaranteed to fit the new PowerBook models.

The bulletin stressed that the heat sinks should not be removed. Removing them voids Apple's one-year warranty on the computer, but more importantly, doing so creates internal heat problems. The excess internal heat can result in system crashes and hangs, can cause the computer to run hotter than it is supposed to, and its life span could be seriously shortened.

For now, most 4 MB expansion cards should work, so users may be able to suffer with a little less memory in their PowerBooks. Users who tend to use the machines while near an electrical outlet can even keep the computers plugged in and take advantage of virtual memory. Until third-party developers start producing new expansion cards that fall within Apple's guidelines, though, it will be tough to find 6 MB expansion cards for the new machines.